This application relates to U.S. application Ser. No. 07/938,861 filed Sep. 1, 1992 and Ser. No. 07/994,101 filed Dec. 21, 1992, the subject matter of which is incorporated by reference herein.
The present invention relates to a rear projection type image display apparatus and a transmission type screen for use therein.
A rear projection type image display apparatus such as, for example, a television receiver wherein images displayed on a small-sized image generation source, e.g. a projection type cathode-ray tube, are enlarged by a projection lens and then projected on a transmission type screen from the back of the screen, has recently become more and more popular for domestic and business use because of a remarkable improvement of image quality and a large picture plane of the screen permits the viewer to enjoy a powerful attendance feeling.
According to the prior art, in the case where a projection type cathode ray tube is used as an image generation source in such rear projection type image display apparatus, a cathode-ray tube and a projection lens are usually combined together with respect to each of the three primary colors of red, green and blue, and images of the three primary colors are combined on the transmission type screen.
FIG. 25 is a sectional side view showing an internal construction of a rear projection type television receiver for domestic use as a conventional rear projection type image display apparatus.
In FIG. 25, there is shown a transmission type screen 1, a projection type cathode-ray tube of green 7G, a projection lens 8G for the projection type cathode-ray tube 7G, a coupler 9G for coupling the cathode-ray tube 7G and the projection lens 8G with each other, a projection light beam of green 10G, a reflecting mirror 11 and a housing 12. As the projection lens 8G in the rear projection type television receiver for domestic use as the conventional rear projection type image display apparatus, there is mainly used a projection lens of a combined construction involving a convex glass lens element and a methacrylic resin lens element such as those disclosed, for example, in Japanese Patent Laid Open Nos. 250916/89, 246512/91 and 276113/91 and U.S. Pat. No. 4,963,007. As the transmission type screen 1, there is mainly used a transmission type screen of a two-sheet construction consisting of a Fresnel lens sheet and a lenticular lens sheet having a light diffusing material, e.g. glass beads, such as those disclosed, for example, in Japanese Patent Laid Open Nos. 117226/81 and 59436/83.
A more detailed description will be provided below about the transmission type screen of a two-sheet construction which has generally been employed in the conventional rear projection type image display apparatus referred to above.
FIG. 26 is a perspective view showing a principal portion of the transmission type screen 1 in the rear projection type image display apparatus illustrated in FIG. 25. In FIG. 26, there is shown the transmission type screen comprising a Fresnel lens sheet disposed on the side of an image generation source (the picture plane of a cathode-ray tube), and a lenticular lens sheet 4 disposed on an image viewing side. Reference numerals 20 and 40 denote base materials of the Fresnel lens sheet 2 and the lenticular lens sheet 4, respectively, each being a transparent thermoplastic resin.
In the base material 40 of the lenticular lens sheet 4 there are dispersed fine particles of a light diffusing material for the scattering of light. In addition to the case where the fine particles of the light diffusing material are dispersed in the interior of the lenticular lens sheet, there also is the case where such fine particles are laminated as a light diffusing layer on the sheet surface. Reference numerals 21 and 22 denote a light incidence surface and a light exit surface, respectively, of the Fresnel lens sheet 2, the light incidence surface 21 being a plane and the light exit surface 22 being in the shape of a convex Fresnel lens.
Reference numeral 41 denotes a light incidence surface of the lenticular lens sheet 4. The light incidence surface 41 comprises a plurality of first vertical lenticular lenses arranged in the horizontal direction of the picture plane of the screen, the first lenticular lenses extending in the vertical direction of the picture plane. Reference numeral 42 denotes a light exit surface of the lenticular lens sheet 4 which comprises a plurality of second vertical lenticular lenses similar in shape and substantially in an opposed relation to the first vertical lenticular lenses of the light incidence surface 41. In the light exit surface 42, a convex portion 43 is formed between adjacent lenticular lenses, and a light absorbing band (black stripe) 16 is laminated as a glossless layer of a black colored material.
In the above conventional transmission type screen, light rays emitted from various points of images displayed on the surface of a projection type cathode ray tube pass through projection lenses (not shown), then enter the light incidence surface 21 of the Fresnel lens sheet 2 and are thereafter converted to substantially parallel light rays by the Fresnel lens of the light exit surface 22, which parallel light rays then enter the lenticular lens sheet 4.
The light rays thus incident on the lenticular lens sheet 4 then travel toward a focal point near the second vertical lenticular lens faces on the light exit surface 42 under the action of the first vertical lenticular lenses on the light incidence surface 41. From that focal point, the light rays diffuse in the horizontal direction of the picture plane of the screen and emerge on the image viewing side while being diffused in both vertical and horizontal directions of the picture plane of the screen by means of the fine particles of the light diffusing material dispersed within the base material 40.
In the above conventional transmission type screen, the directivity in the vertical direction of the screen can be enhanced by increasing the amount of the light diffusing material dispersed in the base material of the lenticular lens sheet 4.
On the other hand, in the above conventional transmission type screen, if there is an extraneous light, e.g. illuminating light, about half of the extraneous light will enter the light absorbing band 16 provided on the light exit surface 42 of the lenticular lens sheet 4. Most the light rays thus incident on the light absorbing band 16 are absorbed by the light absorbing band 16 other than only a portion thereof which are diffused and reflected.
A further description will be provided below concerning the manner in which the light rays incident on the transmission type screen 1 are diffused by the fine particles of the light diffusing material dispersed within the base material 40 of the lenticular lens sheet 4.
FIG. 27 is a vertical sectional view of the transmission type screen 1 shown in FIG. 26, in which reference numeral 14 represent light rays incident on the screen 1.
In the lenticular lens sheet 4', as shown in FIG. 27, the incident light rays 14 travel ahead while being diffused by the light diffusing material 15 in the base 40, and pass out from the light exit surface 42. Therefore, a light ray width, d, as viewed from the image viewing side is larger than the width of the incident light rays 14, so that the scanning line width or picture element size on the light exit surface 42, when images are projected on the transmission type screen 1, becomes larger, and resulting in an image focusing characteristic which is deteriorated and in which the images blur.
Further, in the lenticular lens sheet 4', the incident light rays 14 are not only diffused by the light diffusing material 15 in the base 40 but also scattered, so a portion of the light rays are reflected again toward the light incidence surface 41 or become stray light rays in the lenticular lens sheet 4', or are absorbed by the light absorbing band 16, thus not reaching the focal point near the light exit surface 42 and hence not emerging from the light exit surface 42, with the result that the brightness of the picture plane of the screen viewed from the image viewing side is deteriorated.
Of the incident light rays 14, those which have been scattered by the light diffusing material 15 and become stray light rays as mentioned above reciprocate as unnecessary reflected light rays within the projection optical system, then a portion thereof eventually reach the light exit surface 42 of the lenticular lens sheet 4', so that the image contrast is deteriorated. Further, when there is an extraneous light such as an illuminating light for example, about half of the extraneous light enters the light absorbing band 16 of the lenticular lens sheet 4, as mentioned above, but the remainder enters the second vertical lenticular lenses of the light exit surface 42 and is diffused and reflected by the light diffusing material 15, thus also causing a lowering of the image contrast.
In the conventional rear projection type image display apparatus such as the conventional rear projection type television receiver using the foregoing projection lenses and transmission type screen, there have been the following problems.
(1) A good image contrast is not obtained. Particularly, in the presence of an extraneous light such as an illuminating light for example, the entire image has a tinge of white color and the contrast is 65 or so, which is markedly inferior to the contrast (about 100) obtained in the absence of extraneous light. This is ascribable to the presence of the light diffusing material in the transmission type screen.
(2) Images blur. This is greatly influenced by the fact that there is substantial aberration in the conventional projection lenses and the conventional transmission type screen light diffusing material.
(3) The directivity in the vertical direction of the picture plane of the screen is narrow. This is because if the amount of the light diffusing material in the transmission type screen is increased for the purpose of enlarging the directivity, the poor contrast mentioned above in item (1) and the image blur mentioned above in item (2) become more marked, and therefore it is impossible to incorporate a large amount of the light diffusing material in the screen. For example, if the directivity in the vertical direction of the picture plane of the screen is enlarged twice, the contrast in the presence of an extraneous light such as an illuminating light becomes 30 or so.
(4) A long depth requires a large occupied area for installation. For example, in the case where the picture plane size is about 40 inches in diagonal length, the depth is required to be about 60 to 70 cm. This is ascribable to a small field angle of projection lens and a long projection distance.
(5) The marginal portion of the picture plane is darker than the center thereof. For example, at the portion near a relative image height of 0.9 in the diagonal direction of the picture plane of the screen, the brightness of the marginal portion is about 25% of that of the picture plane center. This depends on restrictions involved in lens construction in the design of projection lens. The relative image height represents a value obtained by scaling the distance from the picture plane center while assuming half of the diagonal length of the picture plane to be 1.
(6) The appearance of the rear projection type image display apparatus does not provide a high quality impression. Particularly, when no image is projected on the transmission type screen, the entire screen looks to the viewer like a glossless screen of light black color as if ink were applied thereto. This does not provide a good impression. This is because at the viewer's side surface of the screen there is no specular reflection in both the light exit surface and the light absorbing layer, with only diffuse reflection, so there is no gloss.